Down syndrome or trisomy 21 (Ts21) includes congenital heart malformations, usually valvuloseptal abnomalities, which are the leading cause of mortality among infants and young children in this patient population. While the incidence of congenital heart disease in individuals with Down syndrome has been well documented, the molecular mechanisms by which human Ts21 leads to heart malformations are unknown. Based on its expression during cardiogenesis and genetic locus, DSCR1 is a strong candidate for a genetic cause of congenital heart disease of Down Syndrome. DSCR1, encodes a feedback inhibitor of calcineurin signaling and NFAT activation and is expressed in the embryonic heart. NFATc1 is required for valve formation in mice and Preliminary Studies provide evidence for the importance of DSCR1 in normal valvuloseptal development as well as in congenital heart malformations associated with Down syndrome. The experiments in Project 3 will examine the function of DSCR1 in normal and abnormal heart development and assess its contribution to cardiac anomalies in the Ts16 mouse model of Down syndrome. The hypotheses are 1. Cardiac valvuloseptal defects associated with Down Syndrome are caused by aberrant activity of trisomic DSCR1. 2. DSCR1 activity regulated by NFAT transcription factors is required for normal heart valve and septa formation. We propose the following aims: Aim 1. Establish the relationship between DSCR1 expression and valvuloseptal defects in Ts16, NFATc1 deficient and DSCR1 transgenic mice. Aim 2. Determine if trisomy of DSCR1 contributes to Down syndrome-related congenital heart malformations in Ts16 mice. Aim 3. Determine if DSCR1 regulated by calcineurin signaling and NFAT activation controls endocardial cushion remodeling and valvuloseptal development. In vivo genetic and in vitro culture approaches will provide an extensive mechanistic analysis of heart valve formation. The studies in this Project and in the SCCOR have long-term implications for the definition of molecular mechanisms that control normal and abnormal valvuloseptal development as well as for translational studies directed towards valve regeneration and tissue engineering.